The present invention generally relates to cooling devices and more particularly to a cooling device of a high-power laser diode array and a fabrication process of such a cooling device. Further, the present invention relates to a high-power laser diode array that is equipped with such a cooling device.
In high-power solid lasers for use in various industrial applications, it is advantageous to use a laser diode array for optical pumping. By pumping a solid laser by such a high-power laser diode array that produces an output optical beam bundle with a characteristically narrow laser oscillation spectrum, in place of a conventional xenon lamp, an efficient pumping of the solid laser becomes possible.
When a laser diode array is used for such pumping purposes, it is required that the laser diode array is capable of producing the desired high-power laser beam bundle continuously with an optical power of several tens of watts. As such a continuous high-power operation of the laser diode array causes a severe heating therein, an efficient cooling device is indispensable in a laser diode array of such high-power applications. In addition, in order that the use of a high-power laser diode array is accepted in the art of high-power solid lasers, it is necessary to reduce the cost of the laser diode array per unit optical power as much as possible, including the cost of the cooling device.
FIG.1 shows the construction of a conventional cooling device 10 disclosed in the U.S. Pat. No. 5,105,429 for cooling a laser diode array for high-power applications.
Referring to FIG.1, the cooling device 10 includes a lower plate 1 and an upper plate 3 each formed with a channel of cooling water, wherein the lower and upper plates 1 and 3 are assembled so as to sandwich therebetween an intermediate plate 2 made of an insulating material such as a glass slab. The lower plate 1 includes an inlet opening 1A and an outlet opening 1B of the cooling water, while the upper plate 3 is formed with an inlet opening 3A and an outlet opening 3B of the cooling water similarly to the lower plate 1. Further, the top surface of the lower plate 1 carries a branched channel 1C of the cooling water, wherein the channel 1C has a first end in communication with the foregoing cooling water inlet 1A and a plurality of second ends in correspondence to a plurality of branches of the branched water channel 1C.
The intermediate plate 2, on the other hand, is formed, along a front edge 2a thereof, with a slit 2C in correspondence to the foregoing branched second ends of the water channel 1C, wherein the slit 2C acts as a channel of the cooling water flowing across the intermediate plate 2 from a lower side thereof to an upper side thereof. Further, cooling water channels 2A and 2B are formed in the intermediate plate 2 respectively in correspondence to the cooling water inlet 1A and the cooling water outlet 1B.
Further, the upper plate 3 carries, on the bottom surface thereof, micro-channels (not shown) along a front edge 3a of the upper plate 3 in communication with the outlet opening 3B, and the micro-channels are formed with a reduced pitch as compared with the pitch of the water channel 1C.
The upper plate 3 carries on a top surface thereof a laser diode array 4 along the foregoing front edge 3a, and the micro-channels are formed on the bottom surface of the plate 3 right underneath the laser diode array 4.
The lower plate 1, the intermediate plate 2 and the upper plate 3 are assembled with each other as explained already and are fixed by a clamping bolt inserted through aligned central openings 1D-3D, which are formed in the plates 1-3 respectively. Each of the laser diodes in the laser diode array 4 are driven by a driver 5.
In the conventional cooling device 10 of this prior art, it should be noted that the plates 1 and 3 are formed of a single crystal Si substrate, and the channel 1C on the plate 1 as well as the micro-channels on the plate 3 are formed by a photolithographic patterning process that uses a resist process. Thereby, each of the grooves forming the micro-channels on the plate 1 or 3 has a width of about 25 .mu.m and a depth of about 125 .mu.m, and is defined by a crystal surface characteristic to a wet-etching process that is used in the photolithographic patterning process. By using such micro-channels having a very small width, the formation of a boundary layer in the cooling water along the surface of the channel is suppressed effectively and the efficiency of cooling by the cooling water through the micro-channels is enhanced substantially.
In the cooling device 10 of FIG. 1, it should be noted that the photolithographic process used to form the micro-channels requires an expensive exposure apparatus and various associated facilities. Thus, the cooling device of FIG. 1 has a drawback of high production cost. Further, the Si substrate used for the upper and lower plates 1 and 3 or the glass slab forming the intermediate plate 2 is a brittle material, and the cooling device of this prior art suffers from the problem of low yield of production. It should be noted that the front edge 2a of the glass plate 2, which is defined by the slot 2C, is particularly fragile and vulnerable. Because of the mechanical fragileness, the plates 1-3 cannot be tightened when stacked to form the cooling device 10. Thus, the cooling device 10 tends to suffer from the problem of water leakage even when a silicone rubber packing is interposed between adjacent plates. This problem becomes particularly serious in a long-duration operation of the laser diode array.
The cooling device 10 of FIG. 1 further suffers from the problem of increased serial resistance when driving the laser diode array 4 by a driving current that is supplied through the plates 1-3. As the cooling device 10 uses a glass slab for the intermediate plate 2, and because of the fact that a rubber packing material is interposed between the plates 1-3 for eliminating water leakage, it is not possible to supply the drive current to the laser diode array 4 through the plates 1-3, unless a conductor path is provided so as to bypass the plates 1-3.
Thus, it is proposed to provide a metallization layer or a conductive clip on a side wall of the layered body of the plates 1-3 in combination with the use of a conductive rubber packing material in place of using an ordinary insulating rubber packing material for eliminating the water leakage. However, none of these approaches are sufficient to eliminate the problem of increased serial resistance of the laser diode array, and the problem of unwanted Joule heating has been inevitable.
In addition, the cooling device 10 of FIG. 1 has a drawback in that the cooling device 10 does not use the part other than the part where the micro-channels are formed effectively for the cooling of the laser diode array 4. Associated therewith, the efficiency of cooling of the cooling device 10 is not high as is expected.
More specifically, the plate 1 or plate 3, which is formed of Si, has a thermal conductivity substantially smaller than a thermal conductivity of a metal, and thus, the efficient cooling of the laser diode array 4 through the plate 1 or plate 3 by heat conduction is not expected. In addition, no substantial heat conduction is expected through the glass intermediate plate 2. Making things worse, the front edge part 2a of the glass intermediate plate 2 is thermally isolated from the rest of the glass plate 2 by the slot 2C, and thus, no effective cooling is expected for the front edge part 2a, while this front edge part 2a, being located right underneath the laser diode array 4, collects majority of the heat produced by the laser diode array 4.
Thus, the cooling device 10 of FIG. 1 relies solely on the micro-channels for cooling the laser diode array 4, and thus, it is necessary to secure a sufficient surface area for the micro-channels in order to achieve the desired cooling of the laser diode array 4. However, the formation of such a micro-channel structure is expensive as noted before and increases the cost of the cooling device 10. Further, the cooling device 10 is vulnerable to the problem of clogging due to the dust particles contained in the cooling water. Thus the cooling device 10 has a problem of expensive maintenance cost.
In addition, the cooling device 10 of FIG. 1, in which the micro-channels are formed by an anisotropic etching process of the Si substrate, has a drawback in that the degree of freedom of the flow-path pattern is relatively limited. Thus, the cooling water is once divided into a plurality of flows by the micro-channels 1C, while the plurality of flows may merge again in the channel 2C of the intermediate plate 2. Such a merging of the cooling water may cause an inhomogeneous supply of the cooling water in the length direction of the laser diode bar 4. When such a temperature variation occurs, the oscillation wavelength of the laser diode may vary in the length direction. In view of the relatively small thermal conductivity of Si, the variation of the oscillation wavelength appears conspicuously.
FIGS. 2A-2E show the construction of another conventional cooling device 20.
Referring to FIGS. 2A-2E, the cooling device 20 includes a lower lid member 21 formed with a cooling water inlet 21A and a cooling water outlet 21B, on which a lower plate 22, formed with a cooling water inlet 22A and a cooling water outlet 22B in correspondence to the foregoing cooling water inlet 21A and the cooling water outlet 21B, is provided. On the lower plate 22, an intermediate plate 23, formed also with a cooling water inlet 23A and a cooling water outlet 23B in correspondence to the foregoing cooling water inlet 22A and the cooling water outlet 22B, is provided, and an upper plate 24, formed similarly with a cooling water inlet 24A and a cooling water outlet 24B in correspondence to the foregoing cooling water inlet 23A and the cooling water outlet 23B, provided on the intermediate plate 23. Further, an upper lid member 25, formed with a cooling water inlet 25A and a cooling water outlet 25B in correspondence to the foregoing cooling water inlet 24A and the cooling water outlet 24B, provided on the upper plate 24.
It should be noted that the lower plate 22 is formed with a cooling water channel 22C in communication with the foregoing cooling water inlet 22A with a shape that increases in width toward a front edge 22a thereof. On the other hand, the intermediate plate 23 is formed with a slit 23C in the vicinity of a front edge 23a thereof, wherein the slit 23C is isolated from the foregoing cooling water inlet 23A or the cooling water outlet 23B. Thereby, the slit 23C acts as a channel for the cooling water flowing across the plate 23 from the lower side to the upper side of the plate 23.
The upper plate 24 is formed with a micro-channel 24D along a front edge 24a thereof in correspondence to the foregoing slit 23C of the underlying plate 23, wherein the upper plate 24 further includes a cooling water channel 24C in continuation from the micro-channel 24D to the cooling water outlet 24B, with a decreasing width toward to the outlet 24B.
Each of the plates 21-24 is formed of a thermally conductive material such as a Cu plate, and a cooling device is assembled by stacking the plates 21-24 upon each other. In the cooling device thus formed, the cooling water introduced into the inlet 21A reaches the micro-channel 24D after passing through the slit 23C. The cooling water thus reaching the micro-channel 24D absorbs the heat produced by a laser diode array (not shown), which is mounted on the upper lid plate 25 along a front edge 25a thereof. The cooling water is then caused to flow to the cooling water outlet 25B after flowing through the cooling water channel 24C.
FIG.3 shows the micro-channel 24d in detail.
Referring to FIG.3, the micro-channel 24D is formed of a number of parallel ribs 24d formed by a laser machining process. Typically, the ribs 24d are formed so as to define a micro-channel or minute cooling water channel between a pair of adjacent ribs 24b such that the micro-channel thus formed has a width of about 20 .mu.m.
In the cooling device 20 of FIG. 2A-2E, in which the Cu plates 21-25 are stacked, the problem of poor thermal conductivity and electrical conductivity of the plate members forming the cooling device is successfully resolved, contrary to the case of the cooling device 10 of FIG. 1. On the other hand, the cooling device 20 of FIGS. 2A-2E still has a drawback in that each of the plates 21-25 has to be formed one by one by a laser machining process. Thereby, the production cost of the cooling device 20 increases inevitably. While the production cost itself may be reduced by using a wet etching process, a wet etching proceeds isotropically when applied to a metal such as a Cu plate, and the channel thus formed generally has a width more than two times as large as the thickness thereof. In other words, no micro-channel can be formed according to such a wet-etching process. When no micro-channels are formed, the efficiency of cooling of the cooling device is reduced inevitably.
Further, the cooling device 20 tends to suffer from the problem of poor yield of production due to the construction of stacking five or more Cu plates. When staking the Cu plates 21-25 to form a water-tight structure, there arises a problem in that the Cu-plates experience a substantial mechanical deformation as a result of the pressure and heat applied at the time of a diffusion welding process. It should be noted that the part of the Cu plates covering a large opening such as the water channel 22C or 24C experiences a particularly severe deformation. Further, a similar deformation occurs also in the front edge part 23a of the plate 23 where the slit 23C is formed. Further, such a deformation of the cooling device 20 causes a corresponding deformation in the laser diode array provided thereon, and the lifetime of the laser diode array is reduced as a consequence.
Further, in relation to the foregoing deformation of the Cu plates, which tends to cause a collapse in the water channel, the cooling device 20 of FIGS. 2A-2E suffers from the problem of non-uniform cooling of the laser diode array. When this occurs, an unwanted temperature distribution is induced in the laser diode array in the length direction thereof similarly to the case of the cooling device 10 of FIG. 1 as explained already, and the oscillation wavelength of the laser diodes changes variously in the length direction of the laser diode array.
In addition, the cooling device 20, which uses a Cu plate for all of the plates 21-25, has a problem in that the cooling device 20 requires an insulating substrate for carrying the laser diode array. However, the use of such a separate insulating substrate increases the number of parts and hence the number of steps of fabrication of the cooling device 20.
Further, it should be noted that the cooling device 20, which uses a wide water channel 24C in continuation with the micro-channel 24D, suffers from the problem of relatively low cooling efficiency due to the formation of boundary layers in the channel 24C. In other words, the channel 24C does not contribute to the cooling of the laser diode array substantially. Thus, the cooling device 20 also relies primarily upon the micro-channels, and the desired efficient cooling is not achieved. Further, it should be noted that the front edge part 23a of the intermediate plate 23 is thermally isolated from the rest of the Cu plate 23 by the slit 23C. Thereby, the heat transfer from the front edge part 23a to the cooling water via the Cu plate 23 is also not expected. As a result of the use of the micro-channels, the cooling device 20 suffers from the problem of increased fabrication cost and increased maintenance cost, similar to the cooling device 10 of FIG. 1.